When supplying electrical power to a DC load from a three-phase AC power distribution system a conventional six-pulse rectification of the AC will often result in unacceptable current distortion on the power distribution system and a low power factor.
Multiphase transformer rectifier unit is a known technology to achieve near unity power factor and low current distortion when connecting DC loads to AC networks. Conventional multiphase transformer rectifier units comprises a magnetic core with a primary winding set and secondary winding set, a rectifier circuit, and three input terminals for connecting said primary winding set to a three-phase AC power distribution system. The secondary winding set is further arranged to generate multiple three-phase systems at its output, which multiple three-phase systems are phases shifted with respect to each other to provide an increased number of output phases.
With increasingly smaller phase shift between each output phase, i.e. the more AC phases that are rectified, the current distortion decreases correspondingly. Hence, to further reduce current distortion of the power distribution system, more windings have to be added to the secondary winding set to provide more output phases. This will however result in higher conduction losses and increased weight of the unit as the secondary winding set becomes increasingly complex.
Document U.S. Pat. No. 6,930,578 B2 discloses an arrangement comprising two multiphase transformers that are configured to reduce harmonic currents in the power distribution network by a load specific installation of the multiphase transformers. The document discloses first and second multiphase transformer connected to a three-phase AC power source, wherein each of said multiphase transformers feeds an individual AC load. Each transformer comprises a primary having a plurality of sets of contact points for receiving power from a three-phase AC power source, and a secondary electromagnetically coupled to the primary having a single set of contact points for connection to a plurality of loads. Each set of contact points of the primary provides a respective primary-to-secondary phase shift, whereby contacts points of the primary of the first and second transformer are selected after installation of the transformers, such that a harmonic of a current from the primary of the first transformer and a harmonic of a current from the primary of the second transformer substantially cancel. The disadvantages of the arrangement described in U.S. Pat. No. 6,930,578 B2 are that the selection of appropriate contact points of the primary of each of the two transformers is a very complex task that requires suitable measurement equipment. Moreover, transformer cost increases when additional contacts points must be provided to the multiphase transformer.
There is thus a need for an improved multiphase transformer rectifier unit design that provides reduced current distortion on the power distribution system and an increased power factor.